A significant amount of industrial waste water is contaminated with heavy metals such as lead, zinc, copper, antimony, chromium and nickel and radioisotope ions such as radioisotopes of uranium, cobalt, thorium, strontium and cesium. These metals become contaminants in aqueous systems as the result of activities including chemical manufacture, smelting, electroplating, wood treating, industrial and medical use of radioisotopes, etc. When such metals are used, metal discharges in aqueous streams severely damage the environment by posing risk to wild life and human health, and have become a worldwide environmental concern. The successful treatment of low level radioactive effluent also presents a major challenge to the nuclear industry. Therefore, improved methods for removing heavy metals or radioactive isotope ions rapidly and efficiently from contaminated industrial aqueous solutions are highly desired.
Existing metal removal methods include standard, conventional techniques such as evaporation, precipitation, electrolytic techniques, membrane separation, fixed and movable bed ion exchange, and activated carbon purification. However, these methods are not economical or efficient enough in most cases. Zeolites and organically modified smectite clays have also been used in these applications, but there remains a continual need to develop improved materials that are more effective for removal of heavy metals and radioisotopes.
Previous studies have identified natural zeolites, e.g. naturally occurring clinoptilolite, for use in removing heavy metals and radioisotope ions from aqueous solutions. While natural zeolites have been used, they are not sufficiently cost-effective and efficient in removing heavy metals and radioisotope ions.
It has been suggested to use modified zeolites to improve the metal removal efficiency of the natural zeolites. U.S. Pat. No. 5,268,107 (the '107 patent) describes a modified clinoptilolite as an ion exchange material for the removal of radioisotope cations such as the cations of cesium (137Cs) and strontium (90Sr) from an aqueous environment containing radioisotope cations. The modified clinoptilolite of the '107 patent is produced by treating a natural clinoptilolite with sodium hydroxide at a concentration of 0.1 to 5M or with hydrochloric acid at a concentration of from 0.1 to 5M for a treatment time longer than one hour at a temperature of 30° C. to 80° C. The modified clinoptilolite is preferably calcined at a suitable calcining temperature of from 400° C. to 500° C. for a calcining time of at least 3 hours. While the modified clinoptilolite of the '107 patent is improved over unmodified clinoptilolite, it is still desired to have further improvement in the heavy metal or radioisotope ion removal efficiency to have a more commercially attractive product. Applicants have now discovered an improved modified clinoptilolite based ion exchange materials with improved heavy metal and radioisotope ion removal efficiency.